Tire engineers have historically established the contours of tires by building a mold having a predetermined contour, placing green or unvulcanized tires with predetermined amounts of rubber on either side of the reinforcing plies into the mold, expanding the tire casing against the mold using an inflatable bladder and then applying heat and pressure to cure the tire.
The mold is built with annular bead forming rings that form the contour of the bead portion 200 of the tire 100. These bead rings have a molding surface that generally approximates the contour of the rim upon which the tire 100 will fit.
The ply 210 is anchored to the bead cores 200 and conventionally in prior art tires 100 has a working ply path between the bead core 200 and the belt packages 140. At the lower portion of the tire the bead core and the rim flange limits the amount of movement the ply cords 210 can take. At the upper portion of the tire circumferentially extending belt package limits the radial growth of the ply. At a location just under the belt lateral edges 150 the ply cords when tensioned take a contour that approximates a single radius of curvature R.sub.UP. This single radius of curvature R.sub.UP is commonly called the "neutral ply line."
This single radius contour can only be held to a point as shown in FIG. 1. If the contour defined by R.sub.UP is maintained further a very steep curve is achieved that is axially extending at a location well above the rim flange of a conventional truck tire rim.
Ideally, one would like to optimize the ply cord path. One recent disclosure entitled "Tire With Reduce Bead Mass," is disclosed in U.S. Pat. No. 5,526,863. It is suggested that a reduced bead mass of as much as 15% can be achieved by using a small apex with an elongated constant thickness outer component and a large axially outer filler component. The U.S. Pat. No. 5,526,863 patent teaches adding a massive amount of rubber axially outward of the ply turnup. This in combination with the placement of the ply turnup axially inward adjacent the radially outer location of the rim flange effectively enables ply turnup to be moved toward the lower ply curvature above the rim flange such that the gap between the ply turnup and the ply path is a constant T from a distance F1 to below a location F2. This prior art tire had a slight improvement or reduction in tire mass with a corresponding reduction in rolling resistance. The ride performance was either better or worse dependent on the overall physical embodiment tested. Radial and cornering stiffness degraded with the lower dynamic spring rate. High speed capability was also worsened. This inventor seemed to believe that the lighter weight benefits outweighed the durability or performance reductions.
One inherent problem in the above-mentioned tires of the prior art is that the uncured rubber flows during vulcanization making the control of the ply path rather unpredictable.
A second and equally important problem is that the lower ply path must be altered to effectively anchor itself to the bead core. This is particularly true of conventional flange type rims. Those skilled in the art conventionally build a spline fit between the upper sidewall neutral contour and the lower sidewall. The resultant spline occupies the region between the maximum section width and the location radially near the bead flange. The spline fit has a very large radius of curvature often approximating an almost linear segment. In this portion of the ply, the ply path deviates from the neutral ply path, shear stresses develop that can generate heat and degrade the rolling resistance and durability of the tire. The tire of the present invention has a consistent ply path in the lower sidewall that has very low stresses. This lower ply path contour is controlled by a unique precured contoured apex that can minimize or even eliminate the use of a spline fit.